linux-kernel - Re: [PATCH 18/35] mm: Add guard pages around a shadow stack.

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Message-ID: <c1f159ef-a010-c356-e633-66cce859fdd5@kernel.org>
Date:   Fri, 11 Feb 2022 09:54:07 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>
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Subject: Re: [PATCH 18/35] mm: Add guard pages around a shadow stack.

On 2/10/22 15:40, Edgecombe, Rick P wrote:
> On Thu, 2022-02-10 at 15:07 -0800, Andy Lutomirski wrote:
>> On Thu, Feb 10, 2022 at 2:44 PM Dave Hansen <dave.hansen@...el.com>
>> wrote:
>>>
>>> On 1/30/22 13:18, Rick Edgecombe wrote:
>>>> INCSSP(Q/D) increments shadow stack pointer and 'pops and
>>>> discards' the
>>>> first and the last elements in the range, effectively touches
>>>> those memory
>>>> areas.
>>>>
>>>> The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes
>>>> and
>>>> 255 * 4 = 1020 bytes by INCSSPD.  Both ranges are far from
>>>> PAGE_SIZE.
>>>> Thus, putting a gap page on both ends of a shadow stack prevents
>>>> INCSSP,
>>>> CALL, and RET from going beyond.
>>>
>>> What is the downside of not applying this patch?  The shadow stack
>>> gap
>>> is 1MB instead of 4k?
>>>
>>> That, frankly, doesn't seem too bad.  How badly do we *need* this
>>> patch?
> 
> Like just using VM_SHADOW_STACK | VM_GROWSDOWN to get a regular stack
> sized gap? I think it could work. It also simplifies the mm->stack_vm
> accounting.

Seems not crazy.  Do we want automatically growing shadow stacks?  I 
don't really like the historical unix behavior where the main thread has 
a sort-of-infinite stack and every other thread has a fixed stack.

> 
> It would no longer get a gap at the end though. I don't think it's
> needed.
> 

I may have missed something about the oddball way the mm code works, but 
it seems if you have a gap at one end of every shadow stack, you 
automatically have a gap at the other end.